A conventional solenoid valve used in a mass flow controller or the like to control the stroke of a valve element by balance between an attraction force of a fixed core and a restoring force of a plate spring may include for example a solenoid valve 100 of a normal closed type shown in FIG. 2.
This solenoid valve 100 includes a body 101 formed with an inlet passage 102 and an outlet passage 104, and a valve seat 103 projecting upward at an upper opening of the inlet passage 102. A valve sheet 105 which can be brought into contact with or separated from the valve seat 103 is supported, together with a plunger 106, by a plate spring 107. The plate spring 107 is pressed by a retainer 108 of which the peripheral edge is engaged in the body 101. A bonnet 111 serving as a yoke is fitted on the retainer 108. In this bonnet 111, a fixed core 112 is centrally secured. This fixed core 112 is disposed in a coil bobbin 115 around which a coil 113 is wound so that the fixed core 112 protrudes downward from the coil bobbin 115 with a small clearance between the lower end surface of the fixed core 112 and the plunger 106.
This solenoid valve 100 is normally held in a valve-closed state where the valve sheet 105 is pressed against the valve seat 103 by the spring force of the plate spring 107. When current is supplied to the coil 113, the fixed core 112 is magnetized, attracting the plunger 106 upward against the spring force of the plate spring 107. Accordingly, the valve sheet 105 is separated from the valve seat 103 to provide communication between the inlet passage 102 and the outlet passage 104, allowing control fluid to flow therethrough.
When the amount of current to be applied to the coil 113 is changed, an electromaanetic attraction force will accordingly vary. Thus, the amount of stroke of the plunger 106 is changed to adjust the degree of valve opening between the valve seat 103 and the valve sheet 105. It is accordingly possible to control the flow rate of control fluid to be allowed to flow into a secondary side through the outlet passage 104.
[Patent Document 1] Jpn. unexamined patent publication No. 2002-357280